For each metric ton of alkali refined vegetable oil produced in the world approximately 30 kg of soapstock is generated. This is a high potential source of raw material since vegetable oil production is growing, specially soybean oil from Brazil.
Soapstock waste has been used mostly as animal feed, raw material for soap makers, and feed stock for fatty acid production. The existing patents and commercial processes to make fatty acids from soapstock always refers to hydrolysis and acidification steps using strong acids such as sulfuric or hydrochloric acids, producing a mixture of fatty acids, inorganic salts, water, and other small components such as glycerin and phospholipids. Due to the nature of this complex mixture, separation of the crude fatty acids layer representing the organic phase from the aqueous phase is difficult, generally requiring steps such as water washing, settling out, centrifuging, and filtration to separate the other components from the fatty acids. Some novelty has been introduced lately, for instance, the use of potassium soaps which generates lower viscosity feedstock, one of the biggest problem with sodium soaps, as described in the US patent publication 2003/0236422. Another patent disclosing a procedure to make fluid soapstock is described in the U.S. Pat. No. 5,156,879.
The invention is directed to a method for treatment of soapstock obtained by alkali refining of fats to provide a fluid, uniform, pumpable animal feed product. In the method, a raw soapstock is provided. The soapstock is pretreated by adding a strong, soluble base to the soapstock.
The U.S. Pat. No. 6,475,758 discloses the use of an endogenic bacteriaum to acidulate soapstock. It is advantageously acidified by fermentation of endogenous soapstock nutrients and added nutrients under controlled conditions using acidogenic bacteria. The nutrients may include carbohydrate, nitrogen, phosphorous, sulfur from defined or undefined sources. The acidification reaction avoids the use of strong acids for the treatment of soapstock, minimizes wastewater contamination with salts and produces potentially valuable by-products including lactic acid, acetic acid, glyceric acid and nutrient rich microorganisms.
All the above mentioned processes end up with a dark color crude fatty acid having residual moisture and other small molecular weight components. Drying and distillation steps usually are necessary to produce commercial fatty acids to be sold in the marketplace or to use it as esterification feed because impurities are known to lower the esterification reaction rate. Usually, alkaline soapstock is converted into free fatty acids by treatment with strong acids. Afterwards, the fatty acids and water/salts are separated.
The process described in WO 2006/050589 to produce fatty acid esters from soapstock deals with enzymatic esterification, with lipase, of the free fatty acids after acidification of the alkaline soapstock with strong acids. The conversion of triglycerides to fatty acids is not complete and the separation is difficult due to mono-/di-/triglycerides and phospholipids that act as emulsifiers. It was an object of the invention to optimize the conversion of glycerides to fatty acid and to increase the yield of free fatty acids of the soapstock. It was also an object of the invention to minimize the emulsifying effects of the mixture to improve separation steps.